oh, my god. oh, my god. and that's when he came out to see what oh, my god was all about. >> reporter: this san bruno, california, neighborhood is somewhat rare. running through it, a massive, 30-inch, 50-year-old transmission pipeline, 3 feet under the ground, carrying gas under high pressure. only about 7% of those big lines, which carry gas to utilities for description back at homes, run anywhere close to a neighborhood. >> if you look at a per mile, it's pretty rare. it happened to be in a community where it's happened. they happen way too often. >> reporter: 329,000 miles of the big pipes. more than 2 million miles of much smaller pipes that carry gas to your house. we're walking down the middle of the street. >> yes. >> reporter: what's underneath here? >> many pipes, the infrastructure of our city, including gas description line. >> reporter: this is that san bruno looked like. a pipeline carrying gas to the utility.